(1) Field of the Invention
The present invention relates to a method and apparatus for controlling a heater for heating an air-fuel ratio sensor, such as an O.sub.2 sensor. Such a sensor is used in an internal combustion engine for measuring the air-fuel ratio in the exhaust gas.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (for example, an O.sub.2 sensor) system, a based fuel amount is calculated in accordance with the detected intake air amount and engine rotational speed, and the base fuel amount is corrected by an air-fuel ratio coefficient calculated in accordance with the output signal of an air-fuel ratio sensor used to detect the concentration of a specific component, such as oxygen, in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
Note, an output signal of an oxygen-battery-type O.sub.2 sensor, which shows a rich or lean air-fuel ratio, is stable when the temperature of the element of the O.sub.2 sensor is higher than a definite value. That is, the O.sub.2 sensor is in an inactive state when the temperature of the element thereof is lower than a definite value, and the O.sub.2 sensor is in an active state when the temperature of the element thereof is higher than a definite value. When the O.sub.2 sensor is in an active state, it is possible to distinguish whether the air-fuel ratio is rich or lean by comparing the output voltage of the O.sub.2 sensor with a definite value, for example, 0.45 V.
A method of keeping the O.sub.2 sensor in an active state by the incorporation of a heater therein is already known. In the above mentioned O.sub.2 sensor system, if the O.sub.2 sensor is disposed in the exhaust gas flow passage, the heater is turned ON and OFF in accordance with a load parameter of the engine, such as the amount of intake airflow.
Nevertheless, when the heater is turned ON and OFF in accordance with the load parameter of the engine, the heater is frequently switched from ON to OFF or vice versa when the engine is driven at the boundary of a heater ON area and a heater OFF area determined by the load parameter, or when a gear-change is made. This often leads to conditions wherein the heater is frequently switched from ON to OFF or vice versa, and if the number of times that the heater is switched exceeds five hundred thousand within the lifetime of the engine, the wiring of the heater may be damaged beyond repair. In this case, the heater will be rendered inoperable, but the air-fuel ratio will be still controlled according to the output signal of the O.sub.2 sensor, even though the O.sub.2 sensor is in an inactive condition, thus deteriorating the driveability of the vehicle, and the emission characteristics and the fuel consumption of the engine.